(1) Field of the Invention
The invention relates to a circuit assembly for protecting a unit to be operated from a supply network against overvoltage, comprising an input having a first and a second input connection, which are connected to the supply network, and an output having a first and a second output connection, to which the unit to be protected can be connected, and a protection circuit which is provided between the first and the second input connections in order to limit the voltage applied to the protection circuit, according to the preamble of claim 1.
(2) Description of Related Art
From EP 0 431 215 A1, a method and an assembly for protecting a disconnectable thyristor against inadmissible overvoltage are already known. According to the teaching therein, the anode voltage of the disconnectable thyristor is monitored with reference to its cathode voltage and when a predetermined limit value is exceeded, a control signal is generated by means of which the negative voltage source connected to the gate of the disconnectable thyristor is disconnected, and the disconnectable thyristor is turned on by means of a turn-on circuit. In this manner, disconnectable thyristors can be protected against inadmissible overvoltage.
DE 103 38 921 A1 discloses a circuit assembly for protecting against surge voltage. The task therein is based on the problem of protecting devices against surge voltage caused, for instance, by lightning on a voltage network.
The protection circuit provided in this respect comprises, in addition to a limiting device, a switch assembly. The switch assembly comprises a switching element and a control circuit for the switching element, the switching element being configured as a semiconductor device in order to ensure sufficiently precise dimensioning.
The switching device and the limiting device are arranged in series and configured such that the switching device will be conducting given a predefinable first trigger criterion and blocks given a predefined second trigger criterion.
A varistor and/or an ohmic resistance and/or a Zener diode is/are used as the limiting device. The switching element, for example, is realized as a triac and/or diac or as a thyristor or even IGBT (Insulated Gate Bipolar Transistor).
All of the embodiments according to DE 103 38 921 A1 have in common that the limiting element should be realized as a varistor, resistor or a Zener diode.